Aqueous latex paints are in common use, and these paints are frequently applied by brush. It has long been desired to include materials in the paint which will function to improve the brush drag and leveling which are obtained. While reference has been made to paint application by brushing, it will be understood that essentially the same problems effect roller application.
Brush drag is the resistance to motion of the brush as it is drawn across the surface to be painted. Aqueous latex paints are frequently too fluid, and low viscosity paints offer little resistance to brush motion. The result is that the applied coating is too thin, and insufficient paint is applied to cover and hide the surface being painted. Brush drag is improved when the viscosity under the shearing force imposed by the moving brush is increased. With greater brush drag, a greater thickness of paint is applied, and the surface being painted is thus properly covered.
Leveling is the capacity of a paint to flow out and provide a smooth and level surface. When a film of paint is brushed onto a surface with sufficient viscosity to apply a paint layer of proper thickness, the fibers in the brush leave ridges in the applied film. If the paint is too viscous, these ridges remain in the applied film as it dries, and the resulting solid paint film is not smooth and level, as is desired.
As will be evident, one needs increased viscosity to provide adequate brush drag to apply a paint film of proper thickness. At the same time, one needs decreased viscosity in order that the brush marks will flow out and disappear as the applied paint film dries. These apparently opposite characteristics exist under different conditions, the brush drag at high shear and the leveling at low shear. This raises the prospect that both may be improved at the same time.
This invention seeks to provide aqueous latex paints with improved brush drags, especially in combination with improved leveling. These paints can be used to adequately cover a substrate with a single application and will flow out to conceal application irregularities, and this will take place regardless of how the paint is applied, e.g., by brush or by roller.
In the past, extraneous agents were relied upon to provide the desired rheology, to the extent that this was possible with those agents. These post-add rheology modifiers are expensive and introduce ancillary problems of incompatibility, separation, and lack of color acceptance. It would be helpful if the desired rheology characteristics could be incorporated into the aqueous emulsion copolymer which is employed as the resin binder component of the latex paint, and this is an objective of this invention.